Adds SCMI clock protocol support in the SCMI message drivers as
defined in SCMI specification v2.0 [1] for clock protocol messages.

Platform can provide one of the plat_scmi_clock_*() handler for the
supported operations set/get state/rate and others.

scmi_msg_get_clock_handler() sanitizes the message_id value
against any speculative use of clock ID as a index since by
SCMI specification, IDs are indices.

This implementation is based on the OP-TEE project implementation [2]
itself based on the SCP-firmware implementation [3] of the SCMI
protocol server side.

Link: [1] http://infocenter.arm.com/help/topic/com.arm.doc.den0056a/DEN0056A_System_Control_and_Management_Interface.pdf
Link: [2] https://github.com/OP-TEE/optee_os/commit/a7a9e3ba71dd908aafdc4c5ed9b29b15faa9692d
Link: [3] https://github.com/ARM-software/SCP-firmware.git

Change-Id: Ib56e096512042d4f7b9563d1e4181554eb8ed02c
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>